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Introduction:
This page is about the vacuum pump of the brake booster. More information about the brake booster can be found on the brake booster page. That page also contains this same explanation of the vacuum pump.
The required vacuum for a vacuum brake booster is often obtained from the engine vacuum in a petrol engine. A hose runs from the brake booster to the intake manifold. Because there is a vacuum in the intake manifold, vacuum is also drawn from the booster. If the engine is switched off and the brake pedal is pumped several times, the pedal will feel hard. This is because all vacuum has disappeared from the brake booster. When the engine is then started again, the pedal will drop and can be pressed further. This must always be taken into account when a vehicle is being towed; in the car where the engine is not running, 3 to 4 times as much force will need to be applied to the pedal. In that situation, the power steering will also not work. It is therefore wise to drive calmly.
It may occur that the pedal feels hard immediately after switching off the engine; it seems as if the vacuum disappears straight away. This may be due to a cracked vacuum hose between the brake booster and the engine, or a defective non-return valve in the hose. This is usually a round piece of plastic between 2 sections of the hose.
If the hose in question is cracked, it should be replaced as soon as possible. If it tears further or breaks, the complete brake boosting will be lost.
With newer petrol engine technologies (with high-pressure injection / lean mixture), turbo engines and in any diesel engine it is not possible to obtain sufficient vacuum from the intake manifold, because these operate with an excess of air (a maximum amount of air is continuously supplied), which means a separate vacuum pump is required. There are two different vacuum pumps, namely the vane pump and the diaphragm pump. The vane pump is also called the tandem pump or the vacuum pump.
Vane pump:
The vane pump is the most commonly used pump for generating vacuum in the brake booster. This pump is often mounted directly on the rear of the camshaft on the cylinder head, but it can also be driven by the multi / V-belt or alternator.
The operation is as follows: when a vane (red in the image) rotates past the inlet of the pump, the space behind the vane becomes larger. The yellow spring presses the vane against the wall, causing this space to increase further. In the space with the blue arrows, a vacuum is now generated. As the pump continues to rotate, the air (indicated by the red arrow) is discharged to the crankcase or the valve cover.
Diaphragm pump:
The diaphragm pump is placed between the vacuum brake booster and the intake manifold, crankcase or valve cover. The movement of this pump is comparable to the movement of the piston, connecting rod and crankshaft in the car engine. During the downward stroke (left) the space above the diaphragm increases and the valve is sucked downwards. Air now flows from the brake booster (blue) into the pump. When the piston moves up again, the right-hand valve opens (right-hand image). The air is now discharged to the crankcase or the valve cover.
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